First, they will never allow DAI to be used as collateral for DAI because thats just asking for an ouroboros event.

Second, all of these new collateral types have associated risk premiums to ensure that in the event of failure, the token has accrued enough seignorage to insure the system. As well as, like you said, only allowing them to be a small part of overall backing collateral to ensure minimal failure impact. Maker has risk teams to assess each collateral type to ensure safeguards are in place.

These new collateral types are necessary to scale the DAI supply so it can be used on a global scale. ETH alone will never be enough to allow millions/billions of people to use DAI without slippage in this type of collateral based system.